This invention relates to the field of optical communication networks, and in particular to the processing of packet-based optical signals and to the information stored in the packet headers of such signals.
The advantages of packet-based networks are well known. Such systems allow more flexible and efficient use of bandwidth than circuit switched systems.
The recent rapid increase in transmission capacity achieved by optical transmission systems far exceeds the improvements in electronic processing speeds. The conversion of high data rate optical signals into the electrical domain and the processing of such signals provides difficulties and may limit the data handling rates within optical networks. However, this conversion and processing can be required for performing switching and routing functions, and is recognised as causing a restriction.
There have been proposals which provide all-optical networks in which switching and routing take place in the optical domain, thereby avoiding the electrical conversion and processing stages. One proposal involves the use of time-shift keying, but the fine tolerances in timing and delay compensation present serious difficulties.
An alternative proposal is to provide a packet header with information optically encoded at a lower data rate than the data rate of the packet payload. This enables opto-electric conversion circuitry to be employed which has a lower detection bandwidth that that which would be required to carry out opto-electric conversion of the packet payload. Thus, low cost electronics can be used to enable the header to be read for routing purposes, and high speed conversion is required only when the payload data is to be read, at the destination node for the particular signal. The invention is concerned specifically with optical data packets of this type.
One particular problem which arises in asynchronous transmission, such as packet-based transmission, is deriving a clock signal to enable decoding of the packet data. This may be achieved using burst mode receivers or over-sampling receivers, but these are complex and expensive to implement.
According to a first aspect of the present invention, there is provided a structure for an optical packet for transmission over an optical network, comprising a packet header and a digitally encoded packet payload, wherein information is encoded in the header by modulation of a signal comprising a plurality of pulses aligned with regularly occurring timing instants, the payload digits also being aligned with the regularly occurring timing instants, the header information being encoded at a rate lower than the frequency of the timing instants.
This structure enables simplified opto-electric circuitry to be used to read the packet header, and also enables the packet header to be used to derive the clock information required to read the high data rate information from the packet payload. This avoids the need for the initial bits of the packet payload to be used to synchronise the clock. The packet payload may comprise, for example, a 40 Gb/s data sequence, whereas the packet header may comprise a 2.5 Gb/s data sequence.
The packet header may be binary digitally encoded with the bits representing one logical level comprising a plurality of pulses of equal magnitude and the bits representing the other logical value comprising periods of no signal. Preferably, the header is then constrained to include a minimum number of bits comprising pulses of non-zero magnitude. For example, the packet header may comprise an equal number of bits representing the two logical levels.
Alternatively, data bits in the packet header representing one of the two logical levels may comprise a plurality of pulses of equal fist magnitude and the data bits in the packet header representing the other logical level comprise a plurality of pulses of equal second magnitude. This ensures that there is a continuous stream of high data rate pulses in the header for the clock recovery circuit.
The header may be derived from return to zero pulses aligned with the regularly occurring timing instants. Alternatively, the header may be derived from pulses defining a sequence which alternates between digital high and digital low at successive timing instants. This enables non-return to zero encoding to be used. Pulses in the packet header are then spaced at twice the interval of the timing instants (i.e. the digits in the packet payload). This is appropriate if NRZ pulses are used, and the header then comprises a sequence xe2x80x9c . . . 101010 . . . xe2x80x9d (at the payload data rate) in order to provide the equally spaced pulses, but each xe2x80x9c1xe2x80x9d pulse occurs only every second clock cycle at the payload data rate.
According to a second aspect of the present invention, there is provided a method of reading data from an optical packet transmitted over an optical network, the packet comprising a packet header and a digitally encoded packet payload, wherein information is encoded in the header by modulation of a signal comprising a plurality of pulses aligned with regularly occurring timing instants, the payload digits also being aligned with the regularly occurring timing instants, the header information being encoded at a rate lower than the frequency of the timing instants, the method comprising:
using the data in the packet header to activate a clock recovery circuit at the data rate of the data in the packet payload; and
processing the data in the packet payload using the recovered clock signal.
The mean pulse power in the header may be measured, and a decision threshold used in the processing of the data in the packet payload selected as a function of the mean pulse power. This simplifies the determination of the decision threshold. This decision threshold will vary as a function of the intensity of the signal, and this is effectively obtained from the header.
According to a third aspect of the present invention, there is provided an apparatus for reading data form a packet payload of an optical packet transmitted over an optical network, the optical packet comprising a packet header and a digitally encoded packet payload, wherein information is encoded in the header by modulation of a signal comprising a plurality of pulses aligned with regularly occurring timing instants, the payload digits also being aligned with the regularly occurring timing instants, the header information being encoded at a rate lower than the frequency of the timing instants, the apparatus comprising:
an opto-electric conversion circuit; and
a clock recovery circuit, activated by the data of the packet header to derive a clock signal at the data rate of the packet payload, and
a decision circuit which determines whether the payload data signal is above or below a threshold level at instants determined by the clock signal from the clock recovery circuit.
The apparatus may further comprise a circuit for measuring the mean pulse power in the header, and wherein the threshold in the decision circuit is selected as a function of the measured mean pulse power.
The invention also provides for an optical communications network comprising a plural of nodes, wherein data to be transmitted between nodes is encoded in accordance with the invention.